Video frames can be classified as either progressive or interlaced, depending upon the method used to display them. In a progressive frame the horizontal lines of pixels that make up the frame are displayed line by line from top to bottom. In contrast, an interlaced frame is created by displaying two fields in turn, one field (known as the top field) containing the top line of the frame and every second subsequent line, and the other field (the bottom field) containing the second line from the top and every subsequent second line, thus including the bottom line of the frame. Interlaced frames rely on the fact that it takes time for the first field of displayed pixels to decay from the display apparatus, during which time the second field is displayed, so as to create the illusion of a single frame containing all the lines of pixels.
The fields of an interlaced video frame are captured sequentially, which means that there is a small time delay between the first field to be captured and the second field to be captured. It is possible for the information contained in the scene to change in this time interval and for this reason it is desirable that the fields of the video frame are displayed in the correct order.
Interlaced video frames can be described as either “top field first” or “bottom field first”, depending upon which of the fields making up the frame is intended to be displayed first. As there is small delay between displaying the first field and displaying the second field, the field intended to be displayed second may contain different information from that contained in the field intended to be displayed first, for example if movement has occurred in the frame in the delay between displaying the first and second fields. Such differences between the field intended to be displayed first and the field intended to be displayed second are known as “inter-field motion”. If fields containing inter-field motion are displayed in an incorrect order, distortion may appear in the displayed frame. In an interlaced display, for example, the video typically becomes juddery or shaky as information appears earlier than it was intended to appear. In a progressive display, the reversal of the fields will not cause such juddery or shaky video, as the fields are put together and displayed at a rate of N frames per second, rather than 2N fields per second, but regardless of the field order, the inter-field motion will lead to combing artifacts, i.e. areas of the frames where rows of lines appear, giving a “combed” appearance.
The property of a sequence of video frames by which the sequence can be described as either “top field first” or “bottom field first” is referred to as the field dominance (or field polarity) and is generally dictated by the video standards under which the video sequence is either recorded or intended to be displayed. For example, the most popular European broadcast standard is PAL (phase alternating line) and has top field first field dominance, whereas the American broadcast standard is NTSC (national television systems committee) which has bottom field first field dominance. If a video sequence having a particular field dominance is played back through a video system configured to play video sequences of the opposite field dominance, or in other words if the field order is reversed, severe visual artifacts may be produced, for example any motion in the video sequence may have a juddering and jittery appearance. Such artifacts will only occur when the video sequence is displayed on an interlaced display but will not be visible when viewed on a progressive display, as in such a display successive fields are combined together to form a frame for displaying. Although ideally metadata associated with the video stream, which may take the form of a flag encoded in the video stream, will indicate whether a particular video sequence should be top field first or bottom field first, it is possible that either the metadata is not set correctly, possibly as a result of an editing process, or the intended playback equipment, for example video decoder within a digital set top box, is not configured so as to be able to either read the metadata or take the metadata into consideration. It would therefore be beneficial to video producers and broadcasters to be able to quickly and easily determine the field dominance in a video sequence.